The invention relates generally to a disconnect coupling, and more particularly to a self-actuating coupling for disconnecting a high-speed rotating drive shaft from a coaxially aligned driven shaft in case of an over-temperature.
In mechanical or electro-mechanical systems, frequently a driven shaft is powered by a coaxially aligned, abutting drive shaft. Thermal disconnect couplings have been used to protect components powered by the driven shaft by responding to a predetermined high temperature. The couplings, also known simply as xe2x80x9cdisconnects,xe2x80x9d interrupt the torque transfer between the driven components and the source of motive power. That is, these couplings disconnect or disengage the driven shaft from the drive shaft under certain operating conditions.
In aircraft, typical devices that require thermal disconnect protection are auxiliary equipment such as electric generators. Generators are driven by the aircraft main engines or by an auxiliary power unit (APU). Frequently generators are driven through hydromechanical transmissions such as constant speed drives (CSDs) or variable speed, constant frequency (VSCF) drives. The drive and the generator may be integrated, as in an integrated drive generator (IDG). A thermal disconnect is typically disposed between the engine or APU and the CSD or VSCF drive. The disconnect is cooled and lubricated by a fluid, such as hydraulic oil. This oil may be the same as the CSD, generator, or engine gearbox oil, or it may come from a separate supply.
If the equipment overheats, causing the temperature of the lubricating fluid (henceforth xe2x80x9coilxe2x80x9d) to exceed a predetermined value, the disconnect breaks the connection between the engine or APU and the CSD or VSCF drive. In case of malfunction of a generator, such as loss of lubrication or failure of parts, the rotor of the generator may overheat. In addition, inadvertent overfilling of oil may cause overheating. In turn, this may open a dump valve in some generators, which is a safety device. Then the generator condition would change from too much oil to too little oil. This could cause a thermal failure of the generator. If the drive shaft were still transmitting high-speed torque to the generator, it would likely self-destruct and damage nearby equipment as it flew apart.
Thus, it is advantageous to include a thermal disconnect for automatically discontinuing operation of equipment when it overheats. The time the aircraft is out of service is then minimized while the equipment is repaired or replaced.
The drive shaft and the driven shaft are most frequently coupled at the disconnect by teeth, splines, or similar mechanical features. Such disconnects allow one shaft to move axially relative to the other, thereby disconnecting the two shafts from one another when an over-temperature condition occurs. Not only is damage to the drive and the generator thereby reduced, but also secondary damage to surrounding equipment may be prevented. Thus a thermal disconnect may be an important safety device.
In some prior designs, the thermal disconnect was constrained under normal conditions by a mass of eutectic as well as the mechanical features mentioned herein before. The term xe2x80x9ceutecticxe2x80x9d refers to an alloy or solution that melts at the lowest possible, constant temperature. For example, many mixture ratios of lead and tin alloy (solder) are possible, but only one ratio of the two metals is the eutectic. Eutectics also have the desirable property that they transition at a sharp melting point from solid to liquid without becoming plastic or viscous in between. Eutectics may be alloys of other metals, but various alloys of silver, lead, and tin are often used. With those three metals, eutectics can be made with melting points between 179xc2x0 C. and 310xc2x0 C.
Eutectics used in prior disconnects were chosen to have melting points that corresponded with the maximum allowed temperature of the oil. In normal operation, the eutectic was solid. When the predetermined high temperature was exceeded, the eutectic liquefied suddenly and was able to flow elsewhere. This flow then triggered the springs or other mechanical features to actuate the disconnect.
Angularity built into the drive surfaces of splines or clutch teeth, as well as springs, pawls, and solenoids were used in conjunction with eutectics to disconnect a driven shaft from a drive shaft. However, angled splines on the shaft are required to be relatively large to transmit the torque that is involved. Springs require considerable space within the thermal disconnect, typically several cubic inches. Pawls also occupy space, add weight, and work better at speeds lower than typically encountered by aircraft generators. Solenoids actuated manually by a cockpit switch or automatically by sensors have been used as triggers as well. Solenoids may corrode and jam, however, or the wires that energize them may melt from the same over-temperature problem that requires the disconnect to operate.
The use of disconnects for protection implies that components will be cleaned and rebuilt. In some of the prior art, molten material migrated within the disconnect or to adjacent areas after it was actuated. This migration made it difficult to clean up the disconnect and surrounding equipment. In addition, cold flow of relatively soft fusible materials sometimes used was a problem on some designs. That is, the material could migrate without melting because of the forces on it. In such prior designs, the forces were required to be large to ensure that the disconnect would separate under the large frictional loads that could be encountered.
On many modern engines, generators, and gearboxes, the rotational speeds are designed to be higher than they were in decades past. Higher speeds require less torque and smaller, lighter parts. For example, four-cylinder automobile engines idle faster than typical eight-cylinder, larger-displacement engines did some years ago. The same is true of aircraft components. Many aircraft generators and some CSD and VSCF drives take advantage of high drive speeds. A thermal disconnect should therefore operate optimally for high-speed shafts, and in fact take advantage of the high rotational speed in its operation.
Accordingly, it is an object of this invention to provide means for disconnecting a driven device from a drive shaft when the device or its lubricating oil exceeds a predetermined temperature.
Another object is to disconnect a driven device from a drive shaft using the centrifugal force of a molten eutectic as a principal actuating force.
Yet another object is to provide a thermal fuse that allows full torque transfer below a predetermined temperature but no torque transfer above that temperature.
Still another object is to create a thermally actuated disconnect without using springs or other mechanical devices to provide the principal actuating force.
It is another object of the invention to create a disconnect that can confine molten eutectic and thereby require less clean up after an over-temperature incident.
It is yet another object to provide a thermal disconnect with simpler and fewer parts.
It is still another object to reduce the size and the weight of the disconnect.
A major step in the invention is the recognition that the centrifugal force of a molten material, preferably eutectic solder, is sufficient to provide the principal force needed to disconnect a driven shaft from a drive shaft.
According to the invention, a drive shaft is disconnected or disengaged from a coaxially aligned driven shaft by the centrifugal force of a molten fusible material, wherein the molten material provides the principal force needed to disconnect the shafts, the material melts at a predetermined temperature, and the material remains confined within the disconnect.
The invention has the benefit that it may prevent the damage to, or destruction of, equipment. The invention may prevent subsequent likely secondary damage to surrounding equipment. The invention is self-actuating, reducing both the chance for human error and for failure of sensors and electrical devices. Thus, the invention may reduce the maintenance and increase the safety of an aircraft.
The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings.